There is no known treatment for yellow fever, and vaccination campaigns are considered crucial to containing outbreaks.

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A boy cries as he receives a vaccine against yellow fever at a public health center in Sao Paulo, Brazil, Tuesday, Jan. 16, 2018. (AP Photo/Andre Penner)

The World Health Organization announced Tuesday that it now considers all of Sao Paulo state at risk for yellow fever, recommending that all international visitors to the state be vaccinated.

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That puts the megacity of Sao Paulo on the list. But Brazil’s Health Ministry said in a statement that it was not changing its own, recently updated map of at-risk areas, which includes only certain parts of the state and city. The ministry said that the WHO’s more-cautious recommendation for foreigners was made in light of the fact that it is impossible to know where visitors might travel once they arrive in Sao Paulo state.

Antonio Nardi, a senior official at the ministry, later told reporters it was the result of an “excess of concern.”

The announcement comes as an outbreak is gathering steam in Brazil during the Southern Hemisphere summer rainy season and just weeks ahead of Carnival, a major draw for foreign tourists. Nardi noted that most Carnival activities happen in cities, not in the forested areas that are of most concern, and so visitors should be safe.

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Since July 2017, 35 cases of yellow fever have been confirmed in Brazil, 20 of them in Sao Paulo state and three in Rio de Janeiro state, according to Health Ministry data released Tuesday. In all, 20 people have died. Yellow fever is spread by the same mosquito that transmits other tropical diseases, including Zika.

Much of Brazil is considered at risk for the virus, but a corridor along the coast was long largely considered safe. Last year, however, Brazil saw an unusually large outbreak of the disease, including in areas not previously thought to be at risk. More than 770 people were infected, and more than 250 died. In response, the WHO began expanding its map of areas of transmission, including adding all of Rio de Janeiro state.

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Brazil rushed to vaccinate millions of people in a massive campaign last year, and it is continuing those efforts this year. Just last week, the Health Ministry announced that a new campaign would vaccinate nearly 20 million people in dozens of cities in Sao Paulo, Rio de Janeiro and Bahia states. Sao Paulo health authorities announced Tuesday they would start vaccinations a few days early and said the campaign would vaccinate about half of the state’s population by the end of next month.

Most of those people will receive a fractional dose of the vaccine — a strategy the WHO recommends to contain ongoing outbreaks that threaten to outrun vaccine supplies.

Nardi declined to say how many doses Brazil has in its strategic stocks, but he said the country has “sufficient vaccine to vaccinate the entire Brazilian population if necessary.” Brazil is a major producer of yellow fever vaccines, but last year it requested 3.5 million doses from international emergency stockpiles.

Brazilian authorities say studies have shown that a fractional dose is effective for at least eight years, though a WHO factsheet only goes as far as saying it is effective for at least a year “and likely longer.” A full dose is generally considered effective for life.

Even before the WHO announcement, Brazilian media were reporting long lines at health centers in Sao Paulo and Rio de Janeiro on Tuesday morning.

“They should have already vaccinated the whole population of this country,” said Roberta Tonelli Ferreira, 61-year-old who visited a health center in Sao Paulo on Tuesday with her granddaughter Valentina, who is 2. “They wait until a person dies, to have who knows how many deaths, for the public to be frightened to start vaccinating.”

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There is no known treatment for yellow fever, and vaccination campaigns are considered crucial to containing outbreaks. Symptoms of the disease include fever, muscle pain, and nausea; some patients also experience abdominal pain, kidney problems and the jaundice from which the disease gets its name.